Eleonore Fröhlich has answered Near Certain

For specific indications (recurrent infections with C. difficile colitis), efficacy has been shown and is performed routinely in renowned hospitals in the United States see: https://www.hopkinsmedicine.org/gastroenterology_hepatology/clinical_services/advanced_endoscopy/fecal_transplantation.htmlIt works less reliably but holds some promise in the treatment of chronic inflammatory bowel diseases (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6421596/).

Karin Amrein has answered Likely

In many countries, many women, men and children are vitamin D deficient, particularly in the winter months. Therefore, an intake of 600 IU for children and 800-2000 IU daily appears reasonable for many, especially from October to April. Vitamin D is particularly helpful for bone/muscle and the immune system.

Karin Amrein has answered Uncertain

We hope so and are working hard on this issue (VITDALIZE study, https://bmjopen.bmj.com/content/9/11/e031083). A low vitamin D level is clearly associated with worse outcomes including higher mortality, higher rates of kidney injury, sepsis and prolonged duration of mechanical ventilation. 

Karin Amrein has answered Uncertain

We do not know, but we hope so. The first appearance of COVID-19 during the winter months in the Northern hemisphere, the higher incidence in non-white populations and worse outcomes would suggest a role of vitamin D. There are very good data that vitamin D reduces acute respiratory infections (https://www.bmj.com/content/356/bmj.i6583) and asthma exacerbations. Recent US data from a large dataset clearly showed a higher rate of COVID-19 positivity in vitamin D deficient individuals.

Karin Amrein has answered Near Certain

Yes, definitely, especially when using medications from companies without proper quality control. Sometimes, manufacturing errors lead to massively overdosed products. We know that prolonged daily dosing > 10,000 IU of vitamin D3 can lead to intoxication.Rarely, underlying vitamin D hypersensitivitiy (i.e. mutations in the vitamin D metabolizing enzymes) can lead to endogenous vitamin D intoxication with low doses or even without vitamin D supplements.